Ring laser gyros are now well known and are similar to those shown and described in U.S. Pat. No. 3,373,650, issued to Killpatrick, and U.S. Pat. No. 3,390,606, issued to Podgorski. In such ring laser gyros, the ring laser can have moveable mirrors that are traditionally used to control the length of the cavity to an integer number of wavelengths of light. Doing so allows the ring laser beams to be operated at maximum intensity. Previously, a random start-up of the analog control circuits could yield an operating position that might be difficult to maintain over a wide temperature range without running out of mirror transducer authority. That is, the mirror position may only be translationally controlled within the operating limits of the transducer design, generally only several wavelengths of the laser beam light.
For ring laser gyros in general, the path length of a ring laser gyro must be kept constant at an integral number of wavelengths of light for proper gyro operation. Large gyros, however, can mode hop, or change their path lengths by an integral number of wavelengths during a mission without serious degradation in output performance. However, in some laser gyros having a closed-loop optical path length, for example 2.0 inches on a side or leg, the laser beam may extinguish (stop operating) when its transducer mirrors are shifted from one mode to another. Of course, the laser beams operating within a ring laser gyro must not be allowed to extinguish since loss of data will result and navigation errors will be incurred. Therefore, it is desirable that ring laser beams acquire a "mode" at power up, and maintain that same mode (or path length) throughout a mission.
The moveable mirrors on a ring laser gyroscope are traditionally positioned by an analog path length control loop that acts to stabilize the gyro cavity path length to an integer number of wavelengths of the laser light. After initially setting the gyro laser cavity to a length equal to "n" wavelengths, the control loop must then maintain that exact cavity length, although arbitrarily chosen, throughout the mission. Subsequent temperature changes will expand or contract the laser block material of the ring laser gyro control loop is intended to position the mirrors to counteract those material changes which directly affect the closed loop optical path of the ring laser gyro.
In some instances, manufacturers of ring laser gyros are finding it difficult to manufacture a gyro that can maintain a constant path length over a wide temperature range. Specifically, the yield may not be as high as they desire.
However, several methods are being used to increase the yield of gyros that can maintain a constant path length over a wide temperature range. They include (1) extending the range of authority of the transducer mirrors to allow a wider range of movement and (2) thermally stabilizing the gyro block to minimize the material changes thereof, thereby minimizing the required compensation. However, these techniques are generally costly, and also have other undesirable weight and size effects.